The invention is a narrow-band phase-locked loop tone decoder using a fast-capture phase-locked loop.
In the prior art, a phase-locked loop tone decoder, disclosed on pages 38-47 of a book, entitled Signetics Linear Phase Locked Loop Applications Book, 1972, uses a second order phase-locked loop. Such a loop includes a phase detector, a low-pass filter, and a controlled oscillator within the loop. The low-pass filter determines both the capture range of the loop and the desired frequency detection range. The capture and detection ranges are coterminous. A synchronous detector is arranged to respond to the input signal of the decoder and to the output of the oscillator for producing a strong output signal when the input signal tone and the oscillator output are in phase with one another.
For such tone decoders, the time required to capture an input signal varies inversely with the pass band of the filter in the second order phase-locked loop. Although the pass bandwidth can be made very narrow by selection of components of the low-pass filter, the speed of capture varies inversely with that bandwidth. Slow operation of the detector results from long time constants created by components required in the narrow-band low-pass filter.
If a signal is to be decoded in a brief period of time within a narrow detection band, such a second order phase-locked loop may not always capture the signal for decoding before the signal is terminated.
Thus, there is a need for a narrow-band tone decoder arrangement which will determine within a brief period of time whether or not an applied signal falls within a predetermined narrow frequency range.
Therefore it is an object to provide a fast-operating narrow-band tone decoder.
It is a further object to rapidly determine whether or not a received tone signal is within a narrow frequency band.
It is another object to provide an improved phase-locked tone decoder.